One of the major problems that has confronted the art in the fabrication of metallic core panels wherein the securement core strips are provided with securement portions constituted by flanges or tabs on the opposite edges of the core strip webs, is the excessive heat generated during the welding process and the resultant disproportionate size of the weld nugget which is created in the face sheets of the panel.
In other words, prior art techniques have necessitated the generation of nuggets whose size is excessive in terms of achieving structural integrity between the face sheets and the honeycomb core structure.
One of the methods previously developed by Campbell and disclosed in the patents enumerated hereinbelow resulted in a completely welded honeycomb core panel which constituted a significant advance over the prior art in that the feasibility of providing a completely welded panel was demonstrated in the art for the first time. The resultant product has achieved considerable commercial success in applications where high sonic fatigue, high temperatures and high buckling loads have prevented successful utilization of the prior art adhesively secured honeycomb-type panels wherein the structural integrity of the panels is achieved by the use of brazing or resinous adhesives.
However, the continuing demands of sophisticated aero-space technology have necessitated confrontation of the problems of excessive weld nugget size and the degradation of the face sheets which are entailed by the generation of weld nuggets whose dimensions are far in excess of those necessitated to achieve the structural integrity of the resultant honeycomb panel.
In fabricating the structures developed by Campbell and utilizing the related methods and apparatus all disclosed in the patents whose numbers appear hereinbelow, we have applied the techniques largely to various stainless steels, such as Inco 718, 316, 347, Inco 625, etc., and titanium.
We have found that the practice of the methods and utilization of the apparatus of Campbell's prior inventions, while achieving the aforementioned structural integrity of the various elements of the metallic, honeycomb core panel, does result in undesirable degradation of the structural characteristics of the face sheets in that the unnecessarily large weld nuggets, because of their external generation, cause degradation of the surfaces of the face sheets and result in the creation of stress risers which reduce the ideal structural characteristics which should be obtained if the large weld nugget size and surface degradation of the face sheets and stress risers could be eliminated.
The aforementioned problems have resulted, in part, from the use of externally positioned welding wheels which apply a translatable weld potential across the face sheets and core flanges to a serrated electrode bar. Since the electrode bar is in contact with the entire length of the core strips, welding current leakage to zones other than those being welded occurs generating excessive heat in the face sheets in order to develop sufficient heat in the internally located securement core strip flanges or tabs to establish structural integrity between said flanges and tabs and the face sheets themselves.
Because of the aforementioned excessively large contact areas of the welding wheels and the leakage and bleeding across the face sheets, excessive current magnitudes are entailed, requiring consequent high pressures between the welding wheels and the internally located projection welding bar, which, in turn, tend to entail the utilization of still higher current magnitudes. Our investigations have indicated that there is an optimum band of current and pressure which will provide a weldment whose nugget dimension eliminates all of the problems alluded to hereinabove.
The methods of the prior art, and the steps followed in performing the same, result not only in face sheet degradation of a large order of magnitude, but in sporadic inadequate welds, burns and melts. Conversely, if the welding current is too low and the pressure too high, no weld will result.
Even when optimum performance of prior art methods and apparatus is achieved where external welding wheels and internal projection electrode bars are utilized, the electrode bars deteriorate rapidly due to the excessive heat and pressure and it has been found that projection-type electrode bars must be replaced after the weldment of approximately 300 to 1000 securement core strips fabricated from easily weldable material.
Added to the cost of replacing the electrode bars themselves are the economic losses inherent in the labor expended in removal and replacement of the electrode bars and contingent upon the machine down-time occasioned while the electrode bars are being replaced in the welding apparatus. The cost of replacing the bars themselves is a major expense.
Another defect of the prior art is the gradual diminishment of weld quality due to the rapid attrition of the projections on the electrode bar, since typical pressures on the projections are approximately 20,000 to 24,000 pounds per square inch. In addition, typical currents are 4000 plus peak amps. This creates indentations on the inner surface of the face sheets and the attendant stress risers in both core flange and face sheets.
Admittedly, satisfactory metallic panels having core structures welded in accordance with the above techniques have been and are being manufactured, but the elimination of the problems mentioned hereinabove results in panels having greater structural integrity which permits use of higher allowables in design.
This is because the utilization of welding wheels conjunction with projection-type electrode bars in the welding process creates welding nuggets whose volume can be as much as fifty times greater than actually needed to structurally integrate the securement portions of the securement core strips to the face sheets with maximum efficiency.
The wheel-projection electrode bar welding concept are disclosed in Campbell U.S. Pat. Nos. 2,930,882; 3,015,715; 3,077,532, 3,498,953 and 3,689,730.